How is skin color measured for the purposes of oximeter performance validation?

According to FDA regulations, manufacturers of pulse oximeters should validate device performance on “a range of skin pigmentations, including at least 2 darkly pigmented subjects or 15% of the subject pool, whichever is larger.” While ‘darkly pigmented’ is not formally defined, it is routinely defined by testing labs as Fitzpatrick skin phototype groups V-VI. The Fitzpatrick scale is a widely used method for classifying skin pigment, from I (pale white) to VI (darkest brown). However, it was originally developed for skin photosensitivity typing, which is not the same as skin color, and should not be conflated with race or ethnicity. Studies have shown inaccuracies in self-reported values, especially in darker skin types. There are other similar visual skin color classification systems, but as with the Fitzpatrick scale, they are all subjective. 

In early 2021, in response to Sjoding et al’s findings of racial (note, not necessarily skin color) bias in pulse oximetry measurement, the FDA issued a statement encouraging further investigation on pulse oximeter accuracy in darker skin types. Given the limitations of the Fitzpatrick scale, there is a need to adopt a standardized objective method for measuring skin color that provides reliable, quantitative, and easily interpretable results. For more discussion on new ways to address racial inequity in oximetry, check out this paper (November 2021).

There are several commonly used skin colorimeters on the market, including Mexameter, Colorimeter, and DermaCatch as listed below. 

Fitzpatrick Scale 

The Fitzpatrick skin type (FST) was developed in 1975 as a tool to assess the likelihood to burn during phototherapy treatments and is commonly used. The original scale was I-IV and it was only later that more diverse skin types were included. 

References: Image: CC BY 3.0 – D’Orazio et al, Int J Mol Sci, 2013; Eilers et al, Accuracy of Self-Report in assessing Fitzpatrick skin phototypes, JAMA Derm, 2013 ; Ware et al, Racial Limitations of Fitzpatrick skin type, Cutis, 2020 ; Wong et al, Analysis of discrepancies between pulse oximetry and arterial oxygen saturation measurements by race and ethnicity and association with organ dysfunction and mortality, JAMA, 2021

Von Luschan’s Chromatic Scale

The Von  Luschan’s Scale was developed as a tool to assess racial classifications according to skin color. It consists of 36 colored tiles which are compared to a person’s skin color.

References: Image: Von Luschan’s Scale

Other ways to quantify skin pigment

Below we describe a few (of the many) products on the market to quantify skin pigmentation.

Mexameter® (MX 18) by Courage & Khazaka

• Narrow-band reflectance colorimeter (only targets melanin and hemoglobin)
• Output: melanin index and erythema index (arbitrary units 0-999)
• Melanin index can be falsely affected by erythema
• Requires base to be plugged in to display results (no computer needed)

Colorimeter® (CL 400) by Courage & Khazaka

• Full visible spectrum reflectance colorimeter
• Output: skin color RGB, L*a*b, xyz, ITA
• Requires USB connection to Windows PC to display results

SkinColorCatch® (previously DermaCatch) by Delfin Technologies

• Full visible spectrum reflectance colorimeter
• Output: skin color L*a*b, ITA, L*c*h, RGB, melanin and erythema indices
• Melanin and erythema indices insensitive to each other
• Portable and battery operated

Interpreting quantitative results

Correlating quantitative measures to a visual color spectrum

Agache 2017 Measuring the Skin, Chapter 6 “The Measurement of Skin Color”:

• Skin color comprises of melanin content, oxy/deoxy-hemoglobin content, and endogenous/exogenous pigments such as bilirubin and carotene
• The CIE L*a*b color space is a device-independent reference for all the colors visible to the human eye and the most commonly used metric to quantify skin color
• Individual Typology Angle (ITA) is calculated from L and b and objectively categorizes skin color into 6 different groups, from very light to dark

Chau et al 2019 Cutaneous Colorimetry as Gold Standard for Skin Color Measurement:

• Explanation of CIE L*a*b color space
• Explanation of how skin colorimeters and spectrophotometers work

Visscher 2017 Skin color and pigmentation in ethnic skin:

• Correlates Fitzpatrick, ITA, Melanin Index

Wilkes et al 2015 Fitzpatrick Skin Type, Individual Typology Angle, and Melanin Index in an African Population: Steps Toward Universally Applicable Skin Photosensitivity Assessments:

• Correlates Fitzpatrick, ITA, Melanin index

Keywords: skin pigment, Fitzpatrick, melanin, skin color, Colorimeter

Additional References: Okunlola et al, Pulse oximeter performance, racial inequity, and the work ahead, Resp Care, Nov 2021; FDA guidance on pulse oximeters; February 19, 2021 FDA statement “Pulse Oximeter Accuracy and Limitations”: addresses Sjoding’s findings, research limitations, and the need for further study; Sjoding et al, Racial Bias in Pulse Oximetry Measurement, NEJM, Dec 2020